Every passenger who flies in American skies is now being checked against a list of people who pose a terrorism risk, Transportation Security Administration head John S. Pistole said Tuesday.

The announcement that 197 air carriers, including 127 that fly internationally, are participating in the screening program brings the TSA into compliance with a key recommendation of the federal commission that investigated the Sept. 11, 2001, attacks.

Pistole said that the TSA is working to refine controversial new pat-down procedures implemented in November. He said his agency wanted to determine whether "less intrusive" methods could be used with confidence that would-be terrorists could be caught.

"Pending that outcome, we're not planning to change anything," he said.

The use of new scanners and requiring enhanced pat-downs for those who refuse to go through them caused an uproar in recent weeks. Surveys show that a majority of Americans - some more unhappy about it than others - are willing to accept the methods as a necessary price to ensure safe travel.

Pistole said that the TSA will work "quickly" to determine whether there is a viable alternative but that he has no timetable. He said the TSA Web site received about 3,300 comments over the weekend about the new procedures, 49 of them complaints.

The TSA began matching passengers against a watch list maintained by the FBI last year. By June, all passengers on domestic flights were being prescreened, and with Tuesday's announcement, all international passengers headed to or from the United States are as well.

The program, known as Secure Flight , is a behind-the-scenes operation designed to ferret out potential terrorists through a multilevel process that begins with the airlines collecting farmore detailed information than in the past when someone buys a ticket.

The airlines forward that information - full name, birth date and sex - to the TSA before the flight. That information is compared against terrorism watch lists. The TSA says that fewer than 400,000 names are on its watch list, that 95 percent of the people are not citizens or lawfully admitted immigrants and that the majority of them are not in the United States.

The TSA says having better passenger data will reduce the number of passengers who are mistakenly identified as terrorists.

"This will reduce, perhaps substantially, the number of people seeking redress" after being mistaken for someone on the watch list, Pistole said. "We haven't seen that yet, but we hope to."

Not everyone on the watch list is banned from flying.

"More than 99 percent of passengers will be able to print out their boarding pass at a kiosk or online," Pistole said. "If they are on a watch list, they'll have to go to the counter agent."

Some will get no farther, but others will be eligible to fly, he said.

Pistole drew a hypothetical case to illustrate the value he sees in Secure Flight. All travel-plan data feed into a single database that provides security officials with a 72-hour view. The screening, however, does not prohibit fliers from buying tickets at the last minute.

He said officials would take note if four or five people on the watch list were heading to the same destination. If two of them were flying on the same plane, air marshals would be added to the flight. And authorities at the destination would be consulted.

"Prior to Secure Flight, we wouldn't be aware we had all those people going to a single location," Pistole said. "It's another intelligence-based layer of security we have before people get to airport security."